In a semi rotary printing process the web substrate moves during image transfer at a printing process speed, decelerates, stops and moves backwards and accelerates for the next printing image. The change in velocity and resulting changes in the tension of the web substrate cause problems, particularly when printing a number of repeating images of a particular size, for example as labels. Variations in substrate tension will cause stretching and contraction of the web substrate and result in images of varying sizes. This process therefore requires a buffering system to compensate for variations in the web flow velocity and a system for controlling the tension of the web. A known passive system is the use of vacuum boxes at the web feed and exit sections of the printing system, in which the vacuum applies tension to the web substrate by pulling a loop of the web substrate towards the back of the vacuum box so that the length of the loop pulled into the box varies to accommodate slack and maintain the tension in the web. However, in such a system there is a varying degree of friction between the walls of the vacuum box and the web substrate that is dependent on the direction of flow of the substrate.